WO2003071285A2

WO2003071285A2 - Apparatus and method for measuring velocity of a projectile in a barrel

Info

Publication number: WO2003071285A2
Application number: PCT/US2003/003915
Authority: WO
Inventors: Christopher J. Lavigna; Justin M. Bowlus; Harry G. Kwatny; Shiping Chen; Sheldon J. Cytron
Original assignee: Techno-Sciences, Inc.; University Of Maryland, College Park; The Government Of The United States As, Represented By The Secretary Of The Army
Priority date: 2002-02-15
Filing date: 2003-02-11
Publication date: 2003-08-28
Also published as: AU2003209094A1; WO2003071285A3; US20030156272A1; US6644111B2; AU2003209094A8; AU2003210942A1; WO2003071286A3; WO2003071286A2; AU2003210942A8

Abstract

An apparatus for measuring velocity of a round in a barrel (32) includes a first optical fiber (12) having first (14) and second (16) fiber optic sensors mounted on the barrel and spaced apart. The sensors are connected to a notch filter (26), which is in turn connected to a photodetector (28). The output of the photodetector is connected to a data processor which calculates the velocity of the round by measuring difference in time between hoop strains caused by the projectile's passing as measured by the sensors and the distance between the sensors.

Description

APPARATUS AND METHOD FOR MEASURING VELOCITY OF A PROJECTILE IN A BARREL

CROSS-REFERENCE TO RELATED APPPLICATIONS

This application claims the benefit of priority of U.S. provisional patent application serial number 60/357,561 filed on February 15, 2002, entitled "Embedded Fiber Optic Sensors for Gun Barrel Diagnostics," the entirety of which is expressly incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST The U.S. Government may have certain rights in this invention. BACKGROUND OF THE INVENTION

The invention relates to measuring the velocity of a projectile in a barrel. The need for measuring the velocity of a round exiting a gun barrel has existed for a number of years. Since the 1980s, with the early development of smart bursting ammunition, a need developed for a way to effectively gauge travel time and detonation point of a round. This problem became more acute with the advanced development of anti-personnel air bursting munitions that require accurate round velocities.

Prior laboratory methods for measuring exit velocity include: yaw screens, inductive rings, foil strain gauges, bore pressure sensors, MEMS (Micro Electro- Mechanical System) accelerometer devices in round, and muzzle velocity radar. Prior field methods for measuring exit velocity include: MEMS devices in round, inductive rings and muzzle velocity radar.

The prior laboratory methods listed above are not suited for field conditions. Yaw screens are inductive wire loops on stands that are set up down range of the projectile near the muzzle. By design they are only useful in range testing situations. Foil strain gauge systems are affected adversely by electromagnetic interference, require frequent calibration, zeroing and adjustment, and do not have the required bandwidth for high velocity projectiles. Bore pressure transducer-based methods require significant modifications to the barrel that are not conducive to field application since they require drilling through the barrel into the bore for insertion of the sensors. This modification weakens the barrel and provides a potential path for water and contaminants to enter the bore. Muzzle velocity radar is expensive, bulky, emits significant electromagnetic radiation markedly increasing the risk of detection, and is typically only able to measure the round exiting velocity for the first round in a burst in automatic mode. MEMS devices in the round are expensive and take up valuable space.

The prior field methods are also unsatisfactory. A fielded inductive ring based system places three heavy inductive rings outboard of the muzzle blast area to measure the round exiting velocity. The addition of this significant extra mass degrades the performance of the gun stabilization system and adversely affects the gun system dispersion. Fielded muzzle velocity radars, similar to their laboratory grade cousins mentioned above, are costly, bulky, emit electromagnetic radiation thereby markedly increasing the risk of detection, and are typically only able to measure the round exiting velocity for the first round in an automatic burst mode.

SUMMARY OF THE INVENTION

The invention provides a method and apparatus for determining the velocity, including, but not limited to, the exit velocity, of a projectile in a gun barrel in a reliable, accurate and cost-effective manner. It accomplishes this for any gun system by measuring strain on the gun barrel using surface mounted or embedded fiber optic sensors. The measurements can be made for single shot or automatic rapid firing modes. The round exit velocity measurement can be used for air burst munitions system applications to enable precise bursting at desired range and improvement of ballistic solutions leading to improved projectile on target performance.

The invention provides a cost effective means to accurately measure the round exit velocity from a broad spectrum of gun barrels. The invention is rugged and can withstand shock, vibrations and high temperatures typically found on gun barrels. The invention is designed to measure high sampling rates of rapid-fire munitions so that each round can be interrogated and given the proper information with regards to time to burst. Temperature compensation as the gun barrel heats up has been incorporated into the invention to maintain accurate velocity measurements during the full firing scenario. The invention will be better understood, and further objects, features, and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals.

Fig. 1 is a schematic drawing of the an apparatus for measuring a velocity of projectile in a gun barrel according to one embodiment of the invention. Fig. 2 is a schematic drawing of some of the components of Fig. 1 mounted on a gun barrel.

Fig. 3 is a plot of optical intensity spectra versus wavelength for the fiber Bragg grating sensors (FBG1 and FBG2) and the fiber Bragg grating notch filter transmission spectrum in an unstrained condition. Fig. 4 is a plot of optical intensity versus wavelength for the fiber Bragg grating sensors and the fiber Bragg grating notch filter with the fiber Bragg grating sensors in a strained condition.

Fig. 5 shows optical output intensity versus time for successive straining of the two fiber Bragg grating sensors. Fig. 6 is a side view of a notch filter mount.

Fig. 7 is a plot of photodetector output voltage versus time for an actual round burst firing.

Fig. 8 A is a plot of barrel surface temperature versus time in seconds.

Fig. 8B is a plot of the output of a first FBG subject to the temperature change shown in Fig. 8A.

Fig. 8C is a plot of the output of a second FBG subject to the temperature change shown in Fig. 8A.

Fig. 9 is a plot of strain for two FBGs versus time in microseconds

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be discussed with reference to preferred embodiments. Specific details, such as types of fiber optic sensors and materials, are set forth in order to provide a thorough understanding of the present invention. The preferred embodiments discussed herein should not be understood to limit the invention. Furthermore, for ease of understanding, certain method steps are delineated as separate steps; however, these steps should not be construed as necessarily distinct nor order dependent in their performance.

When a projectile is ignited, the propellant gases generate very high levels of pressure (>50 ksi) and start to accelerate the projectile (round) down the bore of the gun barrel. As the projectile moves, the gas pressure behind it generates a moving hoop strain wave, i.e., the pressure causes measurable increase in the diameter of the barrel and this disturbance moves with the projectile. Under the influence of these high-pressure gases the projectile reaches very high velocities, up to 1550 m/s or even more for some gun systems. As the projectile nears the muzzle of the barrel, the acceleration slows and a nearly constant velocity is reached. This velocity can be accurately determined from the moving hoop strain wave by measuring the elapsed time for the strain wave to pass each of two fiber optic strain sensors on the barrel that are separated by a known distance. In some embodiments of the invention, the strain sensors are surface-mounted on the barrel. In other embodiments, the sensors are embedded in the barrel. (As used herein, the term "mounting on a barrel" includes mounting a sensor on a surface of the barrel, embedding a sensor in the material of the barrel, and any other method in which a sensor is positioned to be sensitive to strain exerted on the barrel). In those embodiments of the invention that are directed toward measuring exit velocity of the projectile, the sensors are preferably located near the muzzle end of the barrel.

The invention works by timing the passage of the hoop strain wave between a calibrated spacing of fiber strain sensors and computing the velocity by dividing the spacing by the measured elapsed time to obtain the average velocity of the projectile in the barrel between the fiber strain sensors. The projectile may experience a small additional acceleration for a short distance after the last gauge position. If so, a correction factor based on live-fire calibration may be added to the measured velocity to arrive at the actual round exit velocity.

In preferred embodiments of the invention, fiber optic Bragg grating (FBG) sensors are used as the strain sensing elements. An instrumentation system comprising fiber optic and electronic components converts the signals produced by the sensors into useable voltages for post processing. The advantages of using an optical fiber sensor are its ability to withstand both shock and vibration commensurate with live gun firings, as well as to withstand the high temperatures observed in the gun barrel during a typical firing scenario.

Fig. 1 is a schematic drawing of the optical components of one embodiment 10 of the invention. Fig. 2 is a schematic drawing of the components mounted on a gun barrel 32. Referring to Figs. 1 and 2, the apparatus 10 includes a first optical fiber 12 comprising a first fiber optic Bragg grating (FBG) 14 circumferentially attached to the gun barrel 32 at the muzzle 34 of the gun barrel 32. Optical fiber 12 includes a second FBG 16 circumferentially attached to the gun barrel 32 further from the muzzle 34 than the first FBG 14. First FBG 14 and second FBG 16 are separated by axial linear distance L. (Although the FBGs 14, 16 are formed in the same optical fiber in preferred embodiments of the invention, this is not necessary and it will be apparent to those of skill in the art that the FBGs 14, 16 could be formed in separate fibers). The first and second FBGs 14,16 are oriented perpendicular to the longitudinal axis z of the gun barrel. Optical fibers 12, 24 are preferably bonded to gun barrel 32 with, for example, epoxy. It is also possible to mount the sensors by embedding them in the gun barrel rather than surface-mounting them. In preferred embodiments, the FBGs 14,16 are internally formed in the optical fiber 12.

A coupler 18 receives one end of the first optical fiber 12. A light source 20 is connected by a second optical fiber 22 to the coupler 18. Preferably, the light source 20 is a broadband light source. A third optical fiber 24 includes an FBG notch filter 26. The third optical fiber 24 is connected to the coupler 18 to receive light reflected by the FBGs 14,16 in the first optical fiber 12. A notch filter mount 36 is attached to the gun barrel 32 with the FBG notch filter 26 being disposed in the notch filter mount 36. A photodetector 28 is connected to the third optical fiber 24. Data processing electronics 30 are electrically connected to the photodetector 28. Preferably, the first, second and third optical fibers 12,22,24 are single-mode optical fibers. It is noted that the FBG notch filter 26 is internally formed in the optical fiber 24 in this embodiment. The principle of operation of the inventive round exit velocity measurement apparatus 10 depicted in Figs. 1 and 2 involves measurement of the response of the bonded FBGs 14,16 to a measured strain, with associated compensation for temperature fluctuations. Because this embodiment of the invention utilizes the optical technology of Bragg gratings, a shift of the Bragg wavelength, Aλ , in wavelength space is reflective of a thermal and/or a mechanical shift caused by the presence of either a strain or a temperature change, or both. Accordingly, for a Bragg grating sensor bonded to the surface of a structure, the strain and temperature are related to the change in the Bragg wavelength by

Aλ B, _ P_tε + [P_e(a_s - a_f ) + ς] T (1) λ_B

where a and a_} are the coefficients of thermal expansion of the structural material and fiber, respectively, ς is the thermal-optic coefficient, epsilon is the fiber axial strain and P_e is the strain-optic coefficient.

The Bragg gratings 14,16 are oriented so that they are sensitive to hoop strain, preferably circumferentially perpendicular to the barrel axis z around the outer surface of the barrel 32 and bonded in place. The fast rising hoop strain caused by the projectile is transferred from the barrel surface to the FBGs 14,16 and enables detection of the moving projectile as it reaches each FBG 14, 16. Therefore, the hoop strain dilation can be detected as it passes each FBG 14,16 by monitoring the shift in λ_B . By positioning the two strain sensors 14,16 at a fixed separation distance L, at or near the muzzle 34, and measuring the time difference, Δt , of the onset of the shift in λ_B , the round exit velocity can be calculated as:

- ⁼ Δ7t ⁽²⁾

From Equation 2, we can calculate the minimum time resolution, dAt , required for the sensing system to measure the round exit velocity to the required resolution, dV_K I V_κ and express it as dAt = (L/V_re)(dV_K /V (3) Assuming L = 0.25m , V_re = 1400m / s and dV_K = 7.8m / , we can obtain the minimum time resolution of the sensing system as \μs , which translates to a sensor bandwidth of 1 MHz. It is important to note that the round exit velocity measurement requires high speed, not high accuracy, for the two FBGs 14,16 and the detection system. That is, the sensor system needs to be capable of detecting the onset of the shift in λ_B with a resolution of \μs , but does not need to accurately measure the amount of shift.

At the present time, devices that can detect the shift in λ_B directly with the required temporal resolution are either not commercially available or are very expensive. Thus, in order to meet the high-speed requirement, an innovative method for detection of the shift in λ_B is incorporated into some preferred embodiments of the invention. The method comprises employing an FBG notch filter 26 with a precisely defined optical transmission spectrum to suppress the output of the FGBs 14, 16 in the absence of a hoop strain exerted by a projectile passing through the barrel. An example of the spectrum of the FBG notch filter 26 is shown in Fig. 3. In Fig. 3, both FBGs 14,16 are unstrained, thereby not permitting any light to pass through the FBG notch filter 26 to the photodetector 28. The notch filter 26 is designed such that the "notch" in its spectrum contains the peaks of the spectra of the first and second FBGs 14,16, when the FBGs are unstrained. The "notch" of the spectrum of the FBG notch filter 26 has a full width half maximum (FWHM) of about 0.6 nanometers.

When the weapon is fired and the hoop strain dilation reaches each FBG 14,16, the FBGs 14,16 are subjected to a tensile strain stretching, which causes the FBGs to increase the λ_B . As λ_B increases, outside the influence of the FBG notch filter 26, light begins to be transmitted through the FBG notch filter 26 and into the photodetector 28. This effect is shown in Figure 4. Due to the sharp slopes of the FBG notch filter transmission spectrum and the FBGs reflection spectra, even a small shifting of λ_B produces a significantly large change in the light reaching the photodetector 28. Therefore, when the hoop strain dilation reaches each FBG 14,16, a sharp step is seen in the photodetector output current. Figure 5 shows the time response curve from the FBGs as the hoop strain dilation passes both FBGs 14,16. The FBGs 14,16 are sensitive to thermal as well as mechanically induced strains, i.e., thermally and mechanically induced strains both produce a change in the sensor Bragg wavelength, λ_B . Thus, there is a need to provide for temperature compensation to avoid perceiving erroneous signals. If temperature compensation were not provided, heating of the gun barrel due to firing or environmental conditions would lead to erroneous measurements. This would occur because the thermal induced Bragg wavelength shift would cause the peak in the Bragg reflection spectra to move outside the influence of the FBG notch filter 26 (exactly as it would under mechanical induced strain) and light would be transmitted to the photodetector 28. To eliminate this temperature sensitivity, the FBG notch filter center wavelength moves precisely with the peaks in the two barrel mounted FBG spectra as the barrel 32 is subjected to temperature variations. By requiring the FBG notch filter 26 to track with the peaks of the FBGs 14,16, no light will be transmitted to the photodetector 28 as the temperature of the gun barrel 32 varies. To ensure this notch-tracking requirement, the FBG notch filter 26 is located such that it is subjected to the same thermally induced strain as the barrel mounted FBGs 14,16. In one embodiment, the FBG notch filter 26 is disposed in a notch filter mount 36 attached to the surface of the barrel 32. The notch filter mount 36 is designed to have the same temperature expansion characteristics as the barrel itself. That is, the notch filter mount 36 is made of a material having a coefficient of thermal expansion and a thermal conductivity that is substantially the same as the coefficient of thermal expansion and the thermal conductivity of the material of the gun barrel 32. The gun barrel 32 will typically be made of a steel alloy.

Therefore, when the temperature of the surface of the barrel 32 varies as the barrel is subjected to thermal variations such as the heat of firing, the FBG notch filter 26 and the FBGs 14,16 are subjected to the same thermally induced strain. Consequently, the FBG notch filter 26 experiences the same amount of shift in its spectrum as the FBGs 14,16 mounted to the surface of the barrel 32. The relative positions of the FBGs and the FBG notch filter spectra remain unchanged, although the temperature may change. The notch filter mount 36 isolates the FBG notch filter 26 from the hoop strain dilation generated as the projectile passes by the FBG notch filter 26. Fig. 6 is a side view of the notch filter mount 36. The notch filter mount 36 includes clamp portions 38 and 42 having flanges with holes 40 therein for fasteners. The clamp portions 38 and 42 fit around barrel 32 and are fixed in place by fasteners inserted through holes 40. Clamp portion 38 includes a thickened portion 44. The top surface of the thickened portion 44 defines a groove 46 for receiving the FBG notch filter 26. The notch filter mount 36 is attached to gun barrel 32 such that groove 46 is parallel to the longitudinal axis z of the gun barrel 32. The notch filter mount 36 experiences the same temperature as the gun barrel 32. The thickened portion 44 allows the hoop strain to pass without straining the FBG notch filter 26 mounted in groove 46, i.e., the thinner sections of the clamp which are less stiff than the thickened portion 46 absorb essentially all of the hoop strain. The thickness of thickened portion 44 (that is, the thickness from the gun barrel surface to the bottom of groove 46) will vary depending on the caliber of the gun being used.

Multiplexing the FBGs 14,16 into a single fiber 12 results in a single channel system in which the FBGs 14,16 are interrogated independently thereby reducing the need for two channels of sensor signal demodulation optics and electronics. Use of the FBG notch filter 26 results in a simple configuration in which the sharp rising edge of the hoop strain can be detected as a sharp step in light intensity output. A major consideration in the invention is to ensure the repeatability and minimize the complexity of fabrication of the FBG notch filter 26, the optical component that filters the reflected signal from the FBGs 14,16. The FBG notch filter 26 is fabricated using a standard phase mask illuminated by a 244 nm wavelength IR laser. With this fabrication method, the FBG notch filter 26 was produced with the precise FBG notch spectrum required in a low cost repeatable process. The time response of the shift in spectrum of the FBG notch filter 26 and the

FBGs 14,16 due to temperature variation occurs on a much slower time scale than that of the time response of the hoop strain dilation generated by the projectile. This difference in temporal scales of operation of the strain due to the projectile and the strain due to the heating of the barrel may be seen from data in Figs. 8A-C and 9. Fig. 8 A is a plot of temperature versus time in seconds. Fig. 8B is a plot of the output of a first FBG subject to the temperature change shown in Fig. 8 A and Fig. 8C is a plot of the output of a second FBG subject to the temperature change shown in Fig. 8A. Fig. 9 is a plot of strain for two FBGs versus time in microseconds.

As a projectile passes past the FBGs 14,16, pulses of light are generated at the photodetector 28. The photodetector 28 converts the optical signal to an electrical current. The data processing electronics 30 produce an electrical voltage that is amplified and digitized. The data processing electronics include a clock chip that calculates the time difference between the pulses. The velocity is then computed using Equation 2 above.

A test was conducted whereby two FBGs were bonded to a 25 mm bore gun barrel and the gun subjected to a firing burst of ammunition. A muzzle radar system was installed to verify the round exit velocity against the uncompensated values obtained from the uncorrected data. Fig. 7 shows the converted photodetector output, illustrating the periodic nature of the signal corresponding to the passage of each round past the two FBGs. Table 1 shows the derived round velocities from the invention compared to the radar measurement method. Table 1. Results of round exit velocity estimates for multiple round burst tests.

Note: Reference values only available for first round in burst due to Weibel MVR limitation.

In preferred embodiments of the invention, fiber Bragg gratings are used as the fiber optic sensors for measuring hoop strain. However, other types of fiber optic sensors may also be used, including interferometric sensors (e.g., Fabry-Perot cavity sensors) and intensity based sensors (e.g., dual mode sensors). While the invention has been described with reference to certain preferred embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for measuring a velocity of a projectile in a barrel, comprising: a first optical fiber comprising a first fiber optic sensor and a second fiber optic sensor, the first and second fiber optic sensors being mounted on a barrel such that they are sensitive to hoop strain, the first fiber optic sensor being separated from the second fiber optic sensor by a first distance; a coupler that receives one end of the first optical fiber; a light source connected by a second optical fiber to the coupler for supplying light to the first optical fiber; a third optical fiber comprising a notch filter, the third optical fiber being connected to the coupler to receive light from the first optical fiber; a photodetector connected to an end of the third optical fiber; and a processor electrically connected to the photodetector, the processor being configured to determine a difference in time between a hoop strain measured by the first fiber optic sensor and the second fiber optic sensor and to calculate a velocity based on the difference in time and the first distance.

2. The apparatus of claim 1, wherein the fiber optic sensors comprise fiber Bragg gratings.

3. The apparatus of claim 1, wherein the fiber optic sensors are circumferentially mounted to the barrel.

4. The apparatus of claim 1, wherein the fiber optic sensors are surface- mounted on the barrel.

5. The apparatus of claim 1, wherein the fiber optic sensors are embedded in the barrel.

6. The apparatus of claim 1 , wherein the notch filter comprises a fiber

Bragg grating.

7. The apparatus of claim 1, further comprising a notch filter mount mounted to the barrel, wherein the notch filter is disposed in the notch filter mount.

8. The apparatus of claim 7 wherein the barrel is made of a material having a coefficient of thermal expansion and a thermal conductivity and the notch filter mount is made of a material having a coefficient of thermal expansion and a thermal conductivity that is substantially the same as the coefficient of thermal expansion and the thermal conductivity of the material of the barrel.

9. The apparatus of claim 8 wherein the notch filter mount minimizes transmission of hoop strain from the barrel to the notch filter while allowing heat transfer from the barrel to the notch filter.

10. The apparatus of claim 9, wherein the notch filter mount comprises two clamp portions that fit around the barrel, one of the clamp portions including a thickened portion having a top surface that defines a groove therein for receiving the notch filter.

11. The apparatus of claim 10, wherein the groove in the top surface of the thickened portion of the clamp portion is parallel to a longitudinal axis of the barrel.

12. The apparatus of claim 1 , wherein peaks of spectra of the first and second fiber optic sensors, in an unstrained condition, lie within a notch of a spectrum of the notch filter.

13. The apparatus of claim 12, wherein the notch of the spectrum of the notch filter has a full width half maximum of about 0.6 nanometers.

14. The apparatus of claim 1, wherein the first, second and third optical fibers comprise single mode optical fibers.

15. A method of measuring velocity of a projectile in a barrel, comprising: providing a first optical fiber comprising first and second fiber optic sensors mounted on the barrel such that they are sensitive to hoop strain, the first and second fiber optic sensors being oriented perpendicular to a longitudinal axis of the barrel and being spaced apart by an axial linear distance; directing light into the first optical fiber; suppressing outputs of the first and second fiber optic sensors when they are in an unstrained condition using a notch filter; firing a round in the barrel; receiving light reflected from the first and second FBGs in a photodetector; converting the reflected light to an electrical signal in the photodetector; processing the electrical signal to measure a time period for the round to travel the axial linear distance between the first and second FBGs; and computing the exit velocity based on the axial linear distance and the measured time period.

16. The method of claim 15 further comprising the step of mounting the notch filter in a notch filter mount attached to the barrel.

17. The method of claim 16 wherein the notch filter mount minimizes transmission of hoop strain from the barrel to the notch filter while allowing heat transfer from the barrel to the notch filter.

18. The method of claim 16 wherein the notch filter is mounted parallel to the longitudinal axis of the barrel.

19. The method of claim 15 wherein peaks of spectra of the first and second fiber optic sensors, in an unstrained condition, lie within a notch of a spectrum of the notch filter.

20. The method of claim 15 wherein the notch filter comprises a fiber Bragg grating.

21. The method of claim 15 wherein the notch of the spectrum of the notch filter has a full width half maximum of about 0.6 nanometers.

22. The method of claim 15, wherein the fiber optic sensors comprise fiber Bragg gratings.